The present invention relates to demodulation technique of modulated signals, further to particularly effective technique to be applied to demodulation and decoding of received signals in a MODEM having plural modulation systems, and for example effective technique to be utilized for a data communication MODEM or a digital picture data MODEM attached to a facsimile.
Conventionally, in a data communication apparatus such as a facsimile, in order to deal with plural modulation systems, an upper processor to control a modulator-demodulator by upper control protocol or application program changes operation mode of the modulator-demodulator into operation mode of a suitable modulation system every time.